Method of preparation of single crystal ferroelectrics



Sept. 27, 1960 DIELECTRIC CONSTANT s. TRIEBWASSEIR 2,954,300

METHOD OF PREPARATION OF SINGLE CRYSTAL FERROELECTRICS Filed Oct. 51. 1958 FIG.1

TEMPERATURE (0) TEMPERATURE (C) |NVENTOR SOL TRIEBWASSER BYMm/ o TEMPERATURE (c) United Smtes Patent METHOD OF PREPARATION OF SINGLE CRYSTAL FERROELECTRICS SolTriebwasser, Wappingers Falls, N.Y., assignor to International Business Machines Corporation, New York, N.Y., a corporationof New York Filed Oct. 31, 1958, Ser..No'..770,994'

4 Claims. (or. 106-39) material is utilized in conjunctionwith appropriate cir-.

cuitry to achieve a temperature controlling element. The useful range of such an element is limited to the temperature at which there is a sharp break in the dielectric constant versus temperature curve or at the Curie temperature of the material. For single crystals of barium titanate, for example, this temperature is 120 C. For such a device it is extremely useful to provide other single crystal ferroelectric materials having Oun'e points extending over a wide temperature range.

Accordingly, an object of this invention is to provide a method by which a number of single crystal ferroelectric materials exhibiting Curie points extending over a wide temperature range may be conveniently prepared.

Other objects of the invention will be pointed out in the following description and claims and illustrated in the accompanying drawings, which disclose, by way of example, the principle of the invention and the best mode, which has been contemplated of applying that principle.

In the drawings:

Fig. 1 is a phase diagram for the system KNbO KTaO Fig. 2 shows a plot of dielectric constant versus temperature for various solid solutions in the system KNbO KTaO Fig. 3 is a plot of Curie temperature vs. mol percent KTaO for the system KNbO KTaO This invention is based upon a discovery that homogeneous solid solution single crystals of ferroelectric materials of any desired concentration may be prepared directly from a melt containing the individual constituents. The embodiment of the process described herein utilizes to advantage KTaO and KNbO as the individual ferroelectric materials. As shown in Fig. 1, these materials form solid solutions over the entire composition range, which have Curie points varying between 13 K. and 700' K.

According to the process of the present invention, a single crystal of a solid solution of desired composition A may be grown by first providing a melt at temperature C of KTaO and KNbO in proportions given by composition B. The melt is then cooled slowly to point D just below the liquidous temperature B at a rate of about 0.1 C; per hour and maintained at this temperature for some time. During this latter time interval a solid solution single crystal of desired composition A will crystallize from the inelt. A crystal whose mass small in relation to the mass of the charge, preferably about15 should be crystallized during this step. Thereafter the crystal is separated from the melt-and cooled to room temperature over a 3-4 day period', to permit the solid solution single crystal. to assume a more; homogeneous state.

The solid solution constituents may be conveniently formed in situ by mixing Nb 0 Ta Og and K 603 in predetermined amounts, required to form a charge comprising K(Ta Nb,,)O where x ranges from O x 1. Upon reacting these materials at elevated temperatures C0 is evolved and a KTaOy-KNbO solid solution melt is formed. An excess of K 00 may be used" with similar results. In such a ternary system, crystallization will occur' at a slightly lower temperature than from the binary system. During'the crystal growth, a small" seed crystal suspended on a platinum wire in touching contact with the melt may be used to insure good homogeneity in the resultant crystal.

Example.--A charge of 49;80 grams comprising 22.17; grams Nb O (0.0834 mole), 12.26 grams Ta' O (0.0 278 mole) and1'5.37grams K COg (0;l1;l2' mole) and'liaving' the formula KTa Nb O was prepared; ina 60 cc; platinum crucible. The crucible and contents were heated inairat 1-165 CL, cooled to 1-l58 C. during24 hours and maintained at that temperature for an additional 24 hours. A solid solution single crystal formed which was 'then separated from the melt and cooled to room temperature during a 3 to 4 day period. The resultant crystal weighed 2.95 grams and had a specific gravity of 5.84, indicating a concentration of 54 mol percent KTaO or a solid solution having the formula KTa Nb O This composition exhibited a dielectric anomaly at 50 C. and peak dielectric constant was 17 X 10 In a similar manner many different solid solution compositions in the series were prepared. The dielectric properties of these single crystals are also shown in Figs. 2 and 3. The homogeneity of the crystals thus prepared was demonstrated by the character of the plots of dielectric constant versus temperature as shown in Fig. 2. Ceramic materials of the same composition showed rounded peaks at the peak dielectric constant and de creasing values of dielectric constant with increasing concentration of KTaO whereas single crystals exhibited sharp transitions and increasing peak dielectric constants with increasing concentration of KTaO While there have been shown anddescribed and pointed out the fundamental novel features of the invention as applied to a preferred embodiment, it will be understood that various omissions and substitutions and changes in the form and details of the device illustrated and in its operation may be made by those skilled in the art without departing from the spirit of the invention. It is the intention, therefore, to be limited only as indicated by the scope of the following claims.

What is claimed is:

1. A method of preparation of homogeneous solid solution single crystals of ferroelectric materials having Curie points extending over a wide temperature range,-

bulk of the melt, and thereafter cooling said crystal to room temperature over an extended period of time.

2. A method of preparation of homogeneous solid solution single crystals of ferroelectric materials having Curie points extending over a temperature range of from 13 K. to 700 K., comprising the steps of providing a melt of KTaO -KNbO inpredetermined proportions at a temperature just above the liquidous temperature of said material, cooling said melt at a rate of about 0.1 C. per hour to a point just below the liquidous temperature of said melt While maintaining said melt in touching contact With a seed crystal, maintaining said melt at said lower temperature during a time interval in which a single crystal solid solution is formed the mass of which is small in relation to the mass of the melt, separating the crystal thus formed from the bulk of the melt, and thereafter cooling said crystal to room temperature over an extended period of time.

3. A method of preparation of homogeneous solid solution single crystals of ferroelectric materials having Currie points extending over a wide temperature range, comprising the steps of mixing K CO Ta O and Nb O in predetermined proportions, heating said mixture at a temperature just above the liquidous temperature of said material, cooling said melt at a rate of about 0.l C. per hour to a point just below the liquidous temperature of said melt, maintaining said melt at said lower temperature during a time interval in which a single crystal solid solution is formed, the mass of which is small in relation to the mass of the melt, separating the crystal thus formed from the bulk of the melt, and thereafter cooling said crystal to room temperature over an extended period of time.

4. A method of preparation of homogeneous ferroelectric solid solution single crystals of the formula KTa Nb -O having a Curie point of C., comprising mixing K CO Ta O and Nb O in predetermined proportions corresponding to the formula heating said mixture at 1165 C. to form a solid solution melt of said constituents, cooling said melt to 8 C. during 24 hours, maintaining said melt at said latter temperature for an additional 24 hours, separating the crystal thus formed from the bulk of the melt, and cooling the crystal to room temperature during the three-day period.

References Cited in the file of this patent UNITED STATES PATENTS Matthias June 3, 1952 OTHER REFERENCES 

1. A METHOD OF PREPARATION OF HOMOGENEOUS SOLID SOLUTION SINGLE CRYSTALS OF FERROELECTRIC MATERIALS HAVING CURIE POINTS EXTENDING OVER A WIDE TEMPERATURE RANGE, COMPRISING THE STEPS OF PROVIDING A MELT OF KTAO3KNBO3 IN PREDETERMINED PROPORTIONS AT A TEMPERATURE JUST ABOVE THE LIQUIDOUS TEMPERATURE OF SAID MATERIAL, COOLING SAID MELT AT A RATE OF ABOUT 0.1*C. PER HOUR TO A POINT JUST BELOW THE LIQUIDOUS TEMPERATURE OF SAID MELT, MAINTAINING SAID MELT AT SAID LOWER TEMPERATURE DURING A TIME INTERVAL IN WHICH A SINGLE CRYSTAL SOLID SOLUTION IS FORMED THE MASS OF WHICH IS SMALL IN RELATION TO THE MASS OF THE MELT, SEPARATING THE CRYSTAL THUS FORMED FROM THE BULK OF THE MELT, AND THEREAFTER COOLING SAID CRYSTAL TO ROOM TEMPERATURE OVER AN EXTENDED PERIOD OF TIME. 